Window Performance Basics Keeping cool in summer, warm in winter, - - PowerPoint PPT Presentation

Window Performance Basics

Keeping cool in summer, warm in winter, comfortable all the time,... and saving energy too

Ross McCluney, Ph.D., Prinicipal Research Scientist Florida Solar Energy Center

Windows for Energy Smart Buildings

! Many factors affect the design and choice of windows for the Florida home. ! This presentation provides background information

Are windows just “holes in the insulation?”

!Good windows can out-perform opaque insulated walls, energy-wise. !Windows provide much more than energy savings! !A building is there to provide comfort and protection from the elements, not just to save energy. !If energy can be saved too, that’s even better. !We’ll start with some basics !Then we’ll cover energy and economics !And finish with a summary of window option recommendations

Some are,but . . . “it ain’t necessarily so!”

What are windows for?

P Views to the outdoors -visual connections to the natural world P Illumination of the interior with natural daylight P Acoustic connections to the outdoors P Routes for emergency escape P Protection from the discomforts of cold, heat, wind, and rain P Do you see energy anywhere in this list?

Finding the Right Window

P It is more than just choosing a pretty window. P We must also deal with the heat, the cold, as well as the glare and overheating of direct sunlight < The heat and cold: insulation and shading < The glare and overheating of direct sunlight:

• rientation and shading

P Other issues < Choice of window frame and glazing < To insulate or not?

< Acoustic isolation?

< Impact resistance?

< Utility concerns

Dealing with the Sun

P The Good: Big windows provide a bright and open

room with great views and good daylight illumination

P The Bad: Overheating, fading of furnishings,

blocked views

P The Ugly: Killer glare from the sun, big energy

bills, thermal discomfort

P Three strategies for dealing with the sun

< Know where the sun is < Shape and orient the building properly relative to the sun < Shade the windows and walls properly

Factors affecting window options

! Which way the window faces

! How much it is shaded from the sun ! The importance ($-value) of thermal comfort ! The importance ($-value) of sound isolation ! The importance of impact protection ! New construction vs retrofit (replacement) ! Occupant preferences for style and color ! Electric utility company incentives ! Florida Building Code Compliance

Window Fundamentals

• Heat transfers (Radiation, Conduction, Convection)
• The path of the sun through the sky
• Orientation and shading
• Electromagnetic spectrum
• The solar spectrum
• Solar radiant heat gain, direct and diffuse
• Illumination — Daylighting, glare, electric lighting
• The “U-factor” — Conductive heat transfer
• Solar Heat Gain Coefficient (SHGC)
• Visible transmittance (VT)

Subjects to be covered:

Heat Transfer

The three modes of heat transfer

Radiation Conduction Convection Thot Tcold

Heat Flows Through Windows

Absorbed solar radiation conducted through the frame Directly transmitted solar radiation through the glazings (includes both light & heat) Glazing-absorbed solar radiant heat Reflected solar radiation Inward flowing fraction of glazing absorbed radiation Outward flowing fraction of glazing absorbed radiation

Heat Flows Through Windows

Absorbed solar radiation conducted through the frame Directly transmitted solar radiation through the glazings (includes both light & heat) Glazing-absorbed solar radiant heat Reflected solar radiation Heat conducted through the frame Heat conducted through the glass Inward flowing fraction of glazing absorbed radiation Outward flowing fraction of glazing absorbed radiation

Insulated windows reduce conduction, convection, and radiation

Insulation reduces heat conduction through the frame Heat conducted through the glazing system Insulating gas reduces conduction

Warm Cold Hot Cool Winter Summer

Proper spacing minimizes convection Coatings reduce radiation transfer

Knowing Where the Sun is

P Radiation from the sun is generally much stronger than that from the sky, except on hazy and partially overcast days P The sun moves through the sky in a known way each day P Radiation coming directly from the sun’s “disk” is called “direct beam radiation.” P Orienting the building and its windows is important to maximize the benefits and minimize the problems produced by direct beam solar radiation. P First we look at a generic drawing of the sun’s path through the sky on the summer and winter solstices P Then we consider how to orient a house properly relative to the sun’s positions in the sky

Sun rises north of due east, sets north of due west, and is high in the sky at noon Shade:

• verhang for noon

east to northeast morning west to northwest afternoon Sun rises south of due east, sets south of due west, and is low in the sky at noon Shade: southwest to west to protect west window on warm winter days SUMMER WINTER

Orientation and shading

Minimize east and west exposure Shade the facade

N

Wide

• verhangs

Fence

Buffer East and West Exposures

Garage Utility room Closet

Solar Spectrum Fundamentals

P The sun’s radiation covers a range of colors, and beyond. P This electromagnetic radiation has important features for the design and performance of windows in different climates. P We need to know a little more about the physics of solar radiation to fully understand the variety of window products now on the market. P We begin with the electromagnetic spectrum.

Breaking sunlight into its various colors

Red 700 nm Orange Yellow Green Blue

400 nm

Glass prism

Sir Isaac Newton 1723

Invisible infrared Invisible ultraviolet

Electromagnetic Spectrum

Wave - length 1pm 1nm 1:m 1mm 1m 1km Cosmic rays Gamma rays X rays UV Visible spectrum Microwaves Radio IR 400 nm 450 nm 500 nm 550 nm 600 nm 650 nm 700 nm 750 nm Solar spectrum 320 nm 3500 nm UV IR Gamma rays

Parts of the solar spectrum

1.6 0.0

Wavelength in nm

0.2 0.4 0.6 0.8 1.0 1.2 1.4 2500 2000 1500 1000 Solar spectrum Human eye sensitivity (Visible portion of the spectrum)

Near Infrared (NIR)

500 UV VIS NIR

Ultraviolet (UV)

Far Infrared (FIR)

Emission of Heat Radiation

P Warm objects emit radiation P The hotter they are, the more they emit P As their temperature increases, the spectral distribution shifts as well, as shown on the next slide

Warm Objects Emit Radiation

Wavelength in micrometers 0.02 1 10 107 106 105 104 103 102 101 100 10-1 10-2 50 108 0.1

Room temperature Solar Spectral range 0.3 3.5

VIS

FIR

Blackbody radiation spectra from 80 to 35,000 deg Fahrenheit

NIR

Why black body radiation is important

Warm Cold

Warm panes radiate toward cold

• nes

The wave- lengths are in the far IR spectral range We can take advantage of this in designing the glass panes

Spectral Selectivity for Cold Climates

Wavelength

Cold climate glass transmittance Room temperature surface emission spectrum Solar spectrum Human eye response

VIS NIR FIR

Visible light Invisible solar IR Invisible IR emitted by room temperature surfaces

UV

Ultra Violet 200 nm 380 nm 760 nm 3.5 :m 30 :m

Spectral Selectivity for Hot Climates

Wavelength

Hot climate transmittance Cold climate transmittance Room temperature surface emission spectrum Solar spectrum Human eye response

VIS NIR FIR

Visible light Invisible solar IR Invisible IR emitted by room temperature surfaces

UV

Ultra Violet 200 nm 380 nm 760 nm 3.5 :m 30 :m

Quantifying Heat Flows

Ts = Qdirect

Glazing-absorbed solar radiant heat Reflected solar radiation Outward flowing fraction of glazing absorbed radiation

Ni As = Qinward Qg = Ug × Area × )t Eo Eo Eo

Glazing conduction heat transfer Frame conduction heat transfer Visible Transmittance

VT (%) Qf = Uf × Area × )t As = Qabsorbed RsEo Heat flux, Q in W/m2

Incident solar irradiance Transmitted solar radiation

Total glazing solar heat gain

Inward fraction

Eo

Performance Indices

Ts

Glazing-absorbed solar radiant heat Reflected solar radiation Outward flowing fraction of glazing absorbed radiation

Ni As 1

Visible Transmittance

VT As Ts + Ni As = SHGC

U-factor

U (R-value = 1/U) U VT

Primary Indices

Rs

Solar Heat Gain Coefficient

Light to Solar Gain ratio

• A measure of spectral selectivity

VT Visible transmittance: Fraction of incident light transmitted SHGC Solar heat gain coefficient: Fraction of incident solar radiation admitted as heat gain LSG Light-to-Solar Gain ratio: Ratio of visible transmittance to solar heat gain coefficient

LSG = VT SHGC

Spectral Selectivity of Real Glazings

Clear plate Bluegreen #1 Bronze coated Bluegreen #2 Spectrally sel.-1 Spectrally sel.-2

2,500 2,000 500 1,000

1,500

Wavelength in nanometers 0.0 0.2 0.4 0.6 0.8 1.0 Spectral Transmittances of Various Window Glazings

Little Little Mild Strong

Similar IR spectra Lower VT, higher LSG

VIS

VT and SHGC relationships for spectrally selective glazings Visible transmittance

0.0 0.2 0.4 0.6 0.8 1.0

SHGC

0.0 0.2 0.4 0.6 0.8 1.0 LSG = 0.6 0.8 1.0 1.2 1.4 1.6 1.8

SHGC versus VT

Target for hot climate glazings

0.3 0.13 0.33

Single-pane clear glass

SHGC = .58 SHGC = .21

Coatings and Tints

P High solar gain low-e coatings for cold climates P Low solar gain low-e coatings for hot climates P IR-absorbing glass for hot climates P A variety of ways to coat and tint glass One can use

Cold-climate low-e coated windows

Low-emissive configuration

Cold Warm One way to do the job

1

Cold climate glazings Admit and trap solar heat

Insulated gas space (air, argon, krypton) High solar gain low-e coating. Transmits solar, doesn’t emit FIR, so it keeps the heat inside where it is needed Total solar spectrum

FIR

Cold climate glazings Admit and trap solar heat

Cold-climate low-e coated windows

High-reflective configuration Low-emissive configuration

Cold Warm Two ways to do the job

1 2

Cold Warm Cold climate low-e coating.

FIR FIR FIR not emitted FIR reflected

Hot Climate Glazings Admit visible, reject invisible solar

Solar near IR

Warm Cool

Reflective

Hot-climate coated windows

One way to do it

Visible light

1

Hot-climate near-IR reflective coating (Also called “hot-climate low-e coating) (or a low-solar-gain low-e coating)

Visible

• nly

By rejecting nearly half the incident solar radiation with reflection, the SHGC is nearly half as large

NIR

Hot Climate Glazings Admit visible, reject invisible solar

Long-wavelength IR Solar near IR

Cold-climate low-e coating

Solar near IR absorber Warm Cool

Reflective

Hot Cool

Absorptive

Hot-climate coated windows

Two ways to do it

Visible light

1 2

Hot-climate near-IR reflective coating

NIR FIR VIS VIS

Coatings for Energy Control

Cold-climate low-e coated windows

Absorptive longwave conversion High-reflective configuration

Long-wavelength IR Solar near IR Cold-climate low-e coating Hot-climate solar near IR reflective coating

Low-emissive configuration *Second pane optional

in principle Cold Warm Cold Warm Hot Cool Warm Cool Warm Cool Solar near IR absorber (longwave convertor)

*

Hot Cool

Or

Solar direct reflection b. a. *

Hot-climate coated windows

c. d.

Or Or

Putting it all together

Daylight Illumination

PCool, natural daylight has good color rendering PDaylight is healthy PDaylighting can displace electric lighting PElectric utility interactions PThe occupancy schedule is critical

< Florida residential occupancies — are they different?

PDirect beam and diffuse daylight PGlare

< Disability glare < Discomfort glare Glare primer

Disability Glare

The mirror is used to show that if you can see the ceiling luminaire in it, then there is a veiling glare potential. Light from the luminaire reflected from a magazine page will “veil” your view of the text and “disable” your ability to read. This is called disability glare.

Discomfort Glare

With discomfort glare, light from the side that is much brighter than the light from a visual task enters your

• eye. This light

confuses the accommodation mechanisms, producing discomfort, headaches, and premature tiredness.

Direct Beam Solar Radiation

Can produce discomforting glare and localized

• verheating, as well as add

to the air conditioning bill.

Avoiding Direct Beam

Orientation & Shading Strategies

Minimize East and West Exposures Buffer East and West Exposures

N

Garage Utility room Closet Wide

• verhangs

Fence

Window Shading

Outdoors Between the panes Indoors

Exterior window shading strategies

Block solar gain before it reaches the window

When exterior shading is not permitted, desired, or possible

Use High-Performance Glazing Systems

PTo minimize solar heat gain, use hot-climate low-e coated glazings with high LSG ratio P Choose VT to fit the situation

< VT high for north-facing, and exposures already shaded fairly well < VT low for east- and west-facing exposures inadequately shaded

PTo reduce peak load, enhancing comfort and allowing smaller air conditioners, use double pane windows

< Impact resistant for coastal zone < Insulated frames to reduce condensation and improve comfort further

Clear plate Bluegreen #1 Bronze coated Bluegreen #2 Spectrally sel.-1 Spectrally sel.-2

2,500 2,000 500 1,000

1,500

Wavelength in nanometers 0.0 0.2 0.4 0.6 0.8 1.0 Spectral Transmittances of Various Window Glazings

VIS

High VT, low SHGC Medium VT, lower SHGC Low VT, lowest SHGC

Window Energy Performance

PInstantaneous versus long term hourly performance PFor instantaneous perf., get the NFRC label information: U-factor SHGC VT PBut how do you know what are good values of these for your application? PYou need something to tell you about the long-term energy (and peak load) consequences of a given choice PAnd you need a way to convert energy efficiency into economic information. PNext comes some background information on energy computer programs and economic indicators

Hourly Building Energy Simulations

Weather data for each hour

• Air temperature & humidity
• Wind speed
• Direct beam solar
• Global horizontal solar

Assumed internal

heat loads

• Equipment
• Humans & animals
• Occupancy

Building thermal properties

• Thermal mass & location
• Wall, roof, & floor insulation
• Infiltration models
• Window SHGC & U-factors
• HVAC efficiency data

Loads on HVAC system

• Conduction through envelope
• Internal loads
• Fenestration Solar Gain

Other energy consumed

• Equipment
• Electric lighting

Energy use by energy type

• Electric energy
• Electric demand
• Gas energy
• Fuel oil

Dollar costs to operate the

building each hour and for a year

• Annual energy
• Demand charges
• Economic performance indicators

Costs of energy-efficiency

• Building envelope
• HVAC system
• Other features

Window Energy Software

PDOE-2 — Large & complex. Needs engineer to run it. Energy Plus is the next generation. PRESFEN — Easier to run, and based on DOE-2, but you must be somewhat computer savy to run it PEnergyGauge USA — Requires licensing and training PEnergyGauge FlaRes — Used mainly for code compliance PEnergy performance for a typical house can be determined at www.efficientwindows.org but this treats shading only minimally PSample results from DOE-2 on next slide

Example: DOE-2 Results for Miami -1

Lower is better

(Slightly less energy efficient)

(Note: Shading coefficient = 0.87 SHGC)

Example: DOE-2 Results for Miami -2

Lower is better

Example: DOE-2 Results for Miami -3

Conclusions from Miami Example

P U-factor is not that important for annual energy in South Florida P Preventing solar gain is more important P Thus low solar gain single pane, uninsulated windows would appear a good choice for Southern Florida P At least one glass company offers “hard-coat” high LSG glass P But there is more to the study than this. P Further north, insulated windows become more attractive P And there are other benefits of double pane windows: Lowered Peak loads Smaller, less expensive HVAC equipment Acoustic isolation Greater comfort and happiness Motherhood and apple pie

What Can the Homeowner Do to Get Energy Performance Information?

PUse State Building Code energy provisions — Minimal PInsist on NFRC ratings — Instantaneous values only, but still important to know that the numbers are correct PObtain Green Home Certification — Great environmentally, but modest incentive for window energy PUse only Energy Star windows — Good but not best PGuidance for the average homeowner: www.efficientwindows.org/selection3.html PInformation customized for your home, use RESFEN: http://windows.lbl.gov/software/resfen/resfen.html

Florida Building Commission

www.floridabuilding.org/bc/default.asp

Florida Building Code Online View The Florida Building Code Online at SBCCI's website: http://www.sbcci.org/floridacodes.htm Similar sites can be found for many other states in the U.S.

National Fenestration Rating Council

How to Select an Energy Efficient Window

Look for the Energy Star Look for a product that qualifies for the Energy Star in the Northern, Central, or Southern Climate Zone. To distinguish between Energy Star products, go to Step 2. Look for Energy Efficient Window Properties on the NFRC Label The key window properties are U-factor, Solar Heat Gain Coefficient (SHGC), and Visible Transmittance (VT). The NFRC label provides the only reliable way to determine the window properties and to compare products. For typical cost savings from efficient windows in specific locations, go to Step 3. Compare Annual Energy Costs for a Typical House Compare the annual energy use for different window options for a typical 2000-square-foot house in your state or region.

How to Select an Energy Efficient Window

Look for the Energy Star Look for a product that qualifies for the Energy Star in the Northern, Central, or Southern Climate Zone. To distinguish between Energy Star products, go to Step 2. Look for Energy Efficient Window Properties on the NFRC Label The key window properties are U-factor, Solar Heat Gain Coefficient (SHGC), and Visible Transmittance (VT). The NFRC label provides the only reliable way to determine the window properties and to compare products. For typical cost savings from efficient windows in specific locations, go to Step 3. Compare Annual Energy Costs for a Typical House Compare the annual energy use for different window options for a typical 2000-square-foot house in your state or region.

EfficientWindows.org

Windows & Doors Skylights U-Factor 0.75 or below 0.75 or below Maximum Solar Heat Gain Coefficient 0.40 or below 0.40 or below

Energy Star

http://www.energystar.gov/products/windows/

Climatic Zones

Energy Star Homes must meet a performance standard: Have a HERS energy rating of 86 or above Energy Star Windows must meet a prescriptive standard: In the hot climate zone:

Florida Green Home Certification

Florida Green Building Coalition, Inc., www.floridagreenbuilding.org

P Green Home Standard Certification based on a points rating P “Green Home Designation Standard Checklist” publication P Checklist includes points for Energy, Water, Site, Health, Materials, Disaster Mitigation, and a General category P For new homes each category has a minimum number of

• points. The sum of the minimums (default case) is 160.

P Total points requirement is 200. P More points are required if the minimum cannot be met in a category P Window points are given for daylighting, east and west tree shading, and exceeding the Florida Energy Code HERS rating of 80

To Double-pane or not?

PFor energy savings only, double pane is generally not needed in hot climates PIn this case it is more important to put your money into preventing solar gain — On the other hand: PThe highest LSG glass is only available in double pane PDouble pane is more comfortable PDouble pane allows smaller A/C, saving dollars PDouble pane gives better acoustic isolation PThe electric utility might pay you to use double pane (if you ask them nicely) PDouble pane is important for cold climates

Guidance for the Average Building Owner

PPurchase the best window you can afford for your situation, considering:

< Direction the window faces < Degree of existing shading of that window

PShade east- and west-facing windows from direct sunlight

< Trees < Trellis vines < Shrubs and plants < Awnings and shade screens < Shutters

PUse double-pane glass and insulated frames to

< Maintain thermal comfort < Reduce peak A/C size required < Save energy and electricity costs < Protect against possible future peak demand charges

Conclusions - 1

P Our goals should be to < Disconnect from fossil fuel use to the greatest extent possible. < Install very high performance windows, (and very well insulated walls, ceilings, and floors) P A home in the U.S. can drastically reduce its energy requirements and be more comfortable and enoyable as well. P If you are not yet ready to disconnect from your utility, at least strive for maximum fossil energy use efficiency.

Conclusions - 2

PDesigning buildings down to a minimal energy code

< Is a failed opportunity for slowing the growth of energy demand < Ignores comfort, produces more pollution, contributes to global warming < In some cases is not cost-effective even in the traditional economic sense

PDesigning up to greater energy efficiency is a patriotic act—a commitment to the future of humanity and of the Earth.

< It leads to higher quality homes, that are more comfortable and have lower energy bills. < It reduces pollution, lessens global warming, reduces dependence on foreign oil. < It directly contributes to a sustainable future.

PBetter homes attract more customers, permit higher prices, and lead to greater profits for sellers.

Window Recommendations in Summary

P All windows: Insist on high-LSG glazings and double-pane, insulated windows throughout the house—for energy savings, comfort, reduced peak load, and smaller A/C capacity (and cost). P North-facing: Use a side-wall, or a deep window reveal to block low rising and setting sun on hot summer days P South-facing: Use a modest overhang if you like winter sun Use a wide overhang to avoid sun year round High-LSG glazings are especially important if shading’s inadequate P East- and West-facing, a menu of choices: For hot climates: < Dense tree shading where possible Awning shade Exterior shade screen Exterior roller shutters Highest-LSG glazing system, VT between 0.2 and 0.4 Interior reflective operable shade For cold climates: < Well-insulated multiple pane windows with insulated frames P Laminated glass for impact resistance if exterior shade is not enough for this

Additional Information & Resources

# For more information continue exploring our windows web site: www.fsec.ucf.edu/bldg/active/fen/

# For information about the energy crisis: www.dieoff.org # Humanity’s Environmental Future: Making Sense in a Troubled World, by William Ross McCluney, SunPine Press, 219 Johnson Ave., Cape Canaveral, FL 32920 # www.thefutureofhumanity.org